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2-4 Resolution Bandwidth Spectrum Analyzer
2-8 PN: 10580-00349 Rev. M Spectrum Analyzer MG
2-4 Resolution Bandwidth
Resolution Bandwidth (RBW) determines frequency selectivity. The spectrum analyzer traces
the shape of the RBW filter as it tunes past a signal. The choice of resolution bandwidth
depends on several factors. Filters take time to settle. The output of the filter will take some
time to settle to the correct value, so that it can be measured. The narrower the filter
bandwidth (resolution bandwidth), the longer the settling time needs to be, and therefore, the
slower the sweep speed.
The choice of resolution bandwidth will depend upon the signal being measured. If two
closely-spaced signals are to be measured individually, then a narrow bandwidth is required.
If a wider bandwidth is used, then the energy of both signals will be included in the
measurement. Thus, the wider bandwidth does not have the ability to look at frequencies
selectively, but instead simultaneously measures all signals falling within the resolution
bandwidth. Therefore, a broadband measurement would include all signals and noise within
the measurement bandwidth into a single measurement.
On the other hand, a narrow-band measurement will separate the frequency components,
resulting in a measurement that includes separate peaks for each signal. There are
advantages to each. The ultimate decision will depend upon the type of measurement
required.
There is always some amount of noise present in a measurement. Noise is often broadband in
nature; that is, it exists at a broad range of frequencies. If the noise is included in the
measurement, the measured value could be in error (too large) depending upon the noise
level. With a wide bandwidth, more noise is included in the measurement. With a narrow
bandwidth, less noise enters the resolution bandwidth filter, and the measurement is more
accurate. If the resolution bandwidth is narrower, the noise floor will drop on the spectrum
analyzer display. As the measured noise level drops, smaller signals that were previously
obscured by the noise can now be measured.
2-5 Video Bandwidth
Spectrum analyzers typically use another type of filtering after the detector called video
filtering. This filter also affects the noise on the display but in a different manner than the
resolution bandwidth. In video filtering, the average level of the noise remains the same, but
the variation in the noise is reduced. Hence, the effect of video filtering is a “smoothing” of the
signal noise. The resultant effect on the analyzer’s display is that the noise floor compresses
into a thinner trace, while the average position of the trace remains the same.
Changing the video bandwidth (VBW) does not improve sensitivity, but it does improve
discernability and repeatability when making low-level measurements. As a general rule,
most field spectrum analyzer measurements are made at a video bandwidth that is a factor
of 10 to 100 less than the resolution bandwidth. This ratio can be specified in the
BW
main
menu. Using this ratio, with a resolution bandwidth of 30 kHz, typically, the video bandwidth
is set between 300 Hz and 3 kHz, although it can be set anywhere from 1 Hz to 10 MHz.